1. Field of the Invention
The present invention relates to a parallel running control apparatus for pulse-width modulation (hereinafter, PWM) inverters in which a plurality of PWM inverters are run in parallel to perform a variable speed control for an AC motor or the like.
2. Description of the Related Art
FIG. 3 is a configuration diagram of a conventional PWM inverter in which two unit inverters are connected in parallel. In FIG. 3, 10 denotes a first unit inverter. The first unit inverter 10 comprises transistors 10a to 10f which are bridged to one another and diodes 11a to 11f which are connected in an anti-parallel manner with the transistors. 20 denotes a second unit inverter comprising similar transistors 20a to 20f and diodes 21a to 21f. The first and second unit inverters 10 and 20 are connected to a common DC voltage supply E. The output terminals of separate phases of the unit inverters are connected in parallel with one another via interphase reactors 13U to 13W. Current transformers for current detection 12U to 12W and 22U to 22W are provided to detect the output currents of the first unit inverter 10 and second unit inverter 20. The transistors 10a to 10f and 20a to 20f constituting the first and second unit inverters 10 and 20 are connected to a PWM control circuit, which is not shown, for controlling the switching of the transistors.
A parallel running control apparatus, which is not shown, issues three phase AC voltage command values to the PWM control circuit respectively to control the unit inverters 10 and 20 in pulse width modulation. This causes each of the unit inverters 10 and 20 to output AC power with a desired variable voltage and variable frequency. The AC power is then fed to a stator winding of an induction motor or the like.
When the output currents of the unit inverters 10 and 20 are detected by the current transformers 12U to 12W and 22U to 22W, the angular speed of a rotor of the motor is detected at the same time. Based on these detected values, a command value of an angular speed of the rotor, and a command value of an interlinkage magnetic flux, the parallel running control apparatus calculates a secondary magnetic flux, a slip frequency, and other control variables, as well as three phase AC voltage command values.
However, since the output terminals of separate phases of the unit inverters 10 and 20 are connected in parallel with one another via the reactors 13U to 13W, the output currents of the unit inverters 10 and 20 detected by the current transformers 12U to 12W and 22U to 22W include circulating current flowing between the unit inverters 10 and 20. The circulating current poses as an element of disturbance when the parallel running control apparatus calculates the secondary magnetic flux, slip frequency, and other control variables. This results in unstable load control.